In a liquid crystal composition composed of a host liquid crystal and a pleochroic dye dissolved therein as a guest material, it is required for the pleochroic dye to show a high order parameter S (hereinafter sometimes referred to only as "S") in the host liquid crystal.
The order parameter S indicates the parallel degree of the absorption transition moment of dye molecules with respect to the direction of orientation of host liquid crystal molecules, and it is a factor determining a display contrast of a liquid crystal display element. With pleochroic dyes showing parallel dichroism, as the order parameter S approaches 1 which is the maximum value that it can theoretically take, the degree of residual color at whitened areas is reduced, and there can be obtained a light, high contrast, and sharp display. The order parameter S required for pleochroic dyes varies depending on the purpose for which a liquid crystal display element is used and the conditions under which the element is used, and it cannot to determined unconditionally. Usually, however, it is desirable for pleochroic dyes to show an order parameter of at least 0.7 in the vicinity of room temperature when they are used in a host liquid crystal which is suitable for the preparation of the ultimate element.
It is difficult to select pleochroic dyes having a higher S value while showing the desired hue, and it is also very difficult to anticipate such pleochroic dyes from the known dye materials.
Pleochroic dyes which have been developed include merocyanine-, azo- and anthraquinone-based dyes. It is said that anthraquinone-based dyes show generally high stability in a host liquid crystal compared with merocyanine- and azo-based dyes. However, anthraquinone-based dyes having a high S value and giving a Magenta or orange color as a single component have not yet been discovered.